Glass 2. Pisces. 



373 



and are separated by perpendicular plates. A gill bar lies at the 

 inner or oral edge of each of these septa, which for the rest, is spread 

 out by the cartilaginous rays arising from the bar {see p. 364). The 

 outer opening of the cleft or branchial pouch is smaller than 

 the inner; the first lies between the hyoid and the first branchial 

 iirch, those following between arches one and two, two and three, 

 three and four, four and five, respectively. On both anterior and 

 posterior walls, but in the last gill pouch only on the anterior, is a 

 vertical row of fiat, horizontal membranous folds, arranged one above 

 the other ; these are the gill lamellae. Thus the Selachians 

 usually possess nine rows of lamellae on each side, the first on the 

 IDOsterior side of the hyoid, the other eight on the anterior and 

 posterior sides of the first four gill bars. Bach lamella is again beset 

 with fine transverse folds. Besides these five gill pouches there is in 

 most of the Selachians an anterior tubular pouch, the spiracle, 

 between the upper portions of the hyoid and mandibular arches ; it 

 may contain a rudimentary series of gill lamellae, and it opens to the 

 exterior, by a relatively small aperture on the surface of the head. 

 The Cyclostomes resemble the Selachians in the most important 

 respects ; the gill pouches, however, are tubular with a median enlarge- 

 ment; both internal and external openings are small; the lamellse 

 are situated in the widened part. The relations of the gill apparatus 



■c 



Fig. 309. Transverse section of agill arch in various Pish. A Shark, B 

 Chimeera (see p. 384), Sturgeon,!) — B different Teleosteans ; diagrammatic. 

 h gill bar, c septum, s gill rakers. Gill lamellae shaded. — Orig. 



in Ganoidei, Dipnoi, and Teleostei, difi'er considerably 

 from those of the Selachii. In all these groups the five external 

 openings of the gill slits are covered by the operculum, a strong 

 membrane arising from the hyoid, and supported by bony plates and 



